Update agent examples to use claude-sonnet-4.5 and make non-interactive

[Copilot]

Agent examples needed to use claude-sonnet-4.5 as the default model, and both should be non-interactive autonomous demos.

Changes

• Model: Both examples/agent and examples/agent-with-tools now use claude-sonnet-4.5
• agent-with-tools refactor: Converted from interactive stdin loop to autonomous agent demo
  • Replaced CompleteWithTools with ExecuteWithTools for proper agent loop execution
  • Takes task from CLI args or uses default task
  • Runs to completion without user interaction
  • Added exit tool for proper termination

Usage

# examples/agent
go run main.go "List files in the current directory"

# examples/agent-with-tools  
go run main.go "What is the current time and calculate 42 * 17?"

Original prompt

Change the go agent sdk examples to use claude-sonnet-4.5 as the default agent model in the examples. This should only affected the agent examples. Furthremorem while agent example demonstrate a simple loop without interactivigty the agent-with-tools example is an interactive demo. In both cases we need an agent demo - i.e. something that run automatically without interactivity.

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